The basic construction of an axial piston machine of the type in question is disclosed in DE 197 06 263 C1, for example. An axial piston machine of this kind has a cylinder drum which is connected for conjoint rotation to a shaft and is rotatably mounted in a housing. A multiplicity of pistons is guided in the cylinder drum in such a way as to be movable in the axial direction. Piston feet projecting from the cylinder drum in parallel with the axis are supported by respective sliding shoes on a swashplate, the pivoting angle of which can be adjusted in order to set the delivery/absorption volume. The pistons rest against the swashplate via sliding shoes, which are each connected in the manner of a ball joint to the associated pistons and slide in sliding contact with a sliding surface of the non-rotatable swashplate. The sliding shoes are acted upon in the direction of their position of contact with the swashplate by means of a retraction plate, wherein the retraction plate, for its part, is supported on a retraction ball connected to the drive shaft. The retraction plate has a multiplicity of apertures, each of which is penetrated by a sliding shoe collar of the sliding shoe, with the result that the sliding shoes are guided both in the axial direction and in the radial direction.
German Offenlegungsschrift 2 250 510 and German Patent 1 453 452 show axial piston machines of the above-described construction in which the retraction plate is embodied with an encircling circumferential rim which overlaps a segment of the sliding shoes in the axial direction toward the swashplate. By means of this dish-shaped retraction plate, the sliding shoes are covered with respect to the outside.
However, it has been found that considerable churning losses occur with such a design principle when the cylinder is rotating and the housing is filled with oil owing to the revolving sliding shoes, said losses increasing the required drive torque in pump mode and reducing the output torque in motor mode.
To avoid such problems, attempts have already been made to drain the pressure medium from the pump housing by means of a drainage pump. However, such solutions are relatively complex since corresponding design measures and costs for the additional pump arise. Moreover, functional risks are increased since lubrication of components and shaft sealing are not always guaranteed. Another disadvantage is that the power requirement of the additional pump reduces the efficiency of the axial piston machine, meaning that no significant advantage can be achieved over solutions with a housing that is filled with pressure medium.